To move XnviewMP forward to expanded possibilities for searching and filtering XMP metadata, a (simple?) way would be to allow this metadata to be added to the choices of the "Create, File List" tool
This tool already has the options "include sub-folders" and "only image files"

It would then become easy to export a list of records containing the names of the photos and their xmp metadata in csv format, so that you can then create a database with a tool of your choice (Ofiice, Open office...)
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